1. Field of the Invention
The field of the invention is computer equipment for linking a plurality of machine or process controllers in a factory automation network.
2. Description of the Background Art
In factory automation, more complex functions could be performed and coordinated if the programmable or computerized machines presently used to control mechanical equipment could be connected in a network with each other and with higher-level supervisory computers. Current factory automation plans call for at least two levels of supervisory computers for controlling and coordinating the work of programmable controllers. At a lower level, computers known as cell controlling computers or cell controllers monitor and coordinate actions of a number of station-level computers, such as programmable controllers, numerical controllers, motor drive systems, robot controllers and machine vision systems. At a higher level the cell controlling computers communicate with factory-level computers.
The connection of computers in a factory automation system requires improved networking capabilities. In Miller et al., U.S. application Ser. No. 928,529, filed Nov. 7, 1986, an entire database is downloaded from a cell controlling computer to an access machine to provide a remote data cache interface between the cell controlling computer and a group of station-level computers. The access machine reduces message traffic for efficient and fast response of the computer hierarchy to conditions occurring on the factory floor.
The cell controlling computer may execute various application programs which require data from widely distributed, individual stations. The access machine collects data from the station-level devices on the factory floor. The cell controlling computer is tied to the access machine through a data access link. One database operation message transmitted over the data access link may contain data from a number of programmable machines located at various stations along an assembly line. Thus, the cell controlling computer is relieved of a great deal of data collection activity, and this allows for greater speed in execution of its application programs and user interface functions.
The relocation of the database to the access machine allows for a reduction of message traffic to the cell controlling computer. However, the problem of communicating with many stations still exists on the downstream side of the access machine where communication with the stations is handled through local area networks.
Perhaps the most common method of communication in hierarchical computer systems is the polling method in which a higher-level computer sends a "polling" or "request for data" message to a lower-level computer and receives a message in response to the polling message that contains the requested data. It had been considered that a first pair of messages was required to "initialize" the communication with a station, before additional messages could be sent to effect data collection.
To improve performance in multi-tier, hierarchical computer systems, communication must be improved by departing from certain assumptions that have governed communication in earlier systems.